Wireless underground sensor networks (WUSNs) have attracted high attention for their great variety of novel applications, such as underground soil condition and power grid monitoring, mine disaster prevention and rescue, oil gas extraction, earthquake and landslide forecast, border patrol and security, and many more other applications. Unlike typical wireless sensor networks that place wireless sensors in terrestrial environments, WUSNs include wireless sensors in underground environments. The techniques for the typical terrestrial wireless sensor networks may not be (or are not) directly applicable to WUSNs due to the significant differences between the communication media (for example, between the soil and air).